Gas deacidizing methods using aqueous amine solutions are commonly used for removing acid compounds present in a gas, notably carbon dioxide (CO2), hydrogen sulfide (H2S), carbon oxysulfide (COS), carbon disulfide (CS2), sulfur dioxide (SO2) and mercaptans (RSH) such as methylmercaptan (CH3SH), ethylmercaptan (CH3CH2SH) and propylmercaptan (CH3CH2CH2SH). The gas is deacidized by being contacted with the absorbent solution, then the absorbent solution is thermally regenerated.
These acid gas deacidizing methods are also commonly known as “solvent scrubbing”, with a solvent referred to as “chemical”, as opposed to the use of a solvent referred to as “physical” for absorption that is not based on chemical reactions.
A chemical solvent corresponds to an aqueous solution comprising a reactant that reacts selectively with the acid compounds (H2S, CO2, COS, CS2, etc.) present in the treated gas so as to form salts, without reacting with the other non-acid compounds in the gas. After contacting with the solvent, the treated gas is depleted in acid compounds that are selectively transferred as salts into the solvent. The chemical reactions are reversible, which allows the acid compound-laden solvent to be subsequently deacidized, for example under the action of heat, so as to release on the one hand the acid compounds in form of gas that can then be stored, converted or used for various applications, and on the other hand to regenerate the solvent that goes back to its initial state and can thus be used again for a new reaction stage with the acid gas to be treated. The reaction stage of the solvent with the acid gas is commonly referred to as absorption stage, and the stage where the solvent is deacidized is referred to as solvent regeneration stage.
In general, the performances of the separation of acid compounds from the gas in this context mainly depend on the nature of the reversible reaction selected. Conventional acid gas deacidizing methods are generally referred to as “amine methods”, i.e. based on the reactions of the acid compounds with amines in solution. These reactions are part of the overall framework of acid-base reactions. H2S, CO2 or COS are for example acid compounds, notably in the presence of water, whereas amines are basic compounds. The reaction mechanisms and the nature of the salts obtained generally depend on the structure of the amines used.
For example, document U.S. Pat. No. 6,852,144 describes a method of removing acid compounds from hydrocarbons using a water-N-methyldiethanolamine or water-triethanolamine absorbent solution with a high proportion of a compound belonging to the following group: piperazine and/or methylpiperazine and/or morpholine.
The performances of acid gas deacidizing methods using amine scrubbing directly depend on the nature of the amine(s) present in the solvent. These amines can be primary, secondary or tertiary. They can have one or more equivalent or different amine functions per molecule.
In order to improve the performances of deacidizing methods, increasingly efficient amines are continuously sought.
One limitation of the absorbent solutions commonly used in deacidizing applications is insufficient H2S absorption selectivity over CO2. Indeed, in some natural gas deacidizing cases, selective H2S removal is sought by limiting to the maximum CO2 absorption. This constraint is particularly important for gases to be treated already having a CO2 content that is less than or equal to the desired specification. A maximum H2S absorption capacity is then sought with maximum H2S absorption selectivity over CO2. This selectivity allows to maximize the amount of treated gas and to recover an acid gas at the regenerator outlet having the highest H2S concentration possible, which limits the size of the sulfur chain units downstream from the treatment and guarantees better operation. In some cases, an H2S enrichment unit is necessary for concentrating the acid gas in H2S. In this case, the most selective amine is also sought. Tertiary amines such as N-methyldiethanolamine or hindered secondary amines exhibiting slow reaction kinetics with CO2 are commonly used, but they have limited selectivities at high H2S loadings.
It is well known to the person skilled in the art that tertiary amines or secondary amines with severe steric hindrance have slower CO2 capture kinetics than less hindered primary or secondary amines. On the other hand, tertiary or secondary amines with severe steric hindrance have instantaneous H2S capture kinetics, which allows to achieve selective H2S removal based on distinct kinetic performances.
Various documents propose using hindered tertiary or secondary amines, in particular hindered tertiary or secondary diamines in solution, for deacidizing acid gases.
Among the applications of tertiary or secondary amines with severe steric hindrance, U.S. Pat. No. 4,405,582 describes a method for selective absorption of sulfur-containing gases with an absorbent containing a diaminoether at least one amine function of which is tertiary and whose other amine function is tertiary or secondary with severe steric hindrance, the nitrogen atom being in the latter case linked to either at least one tertiary carbon or to two secondary carbon atoms. The two amine functions and the carbons of the main chain can be substituted by alkyl or hydroxyalkyl radicals.
U.S. Pat. No. 4,405,583 also describes a method for selective removal of H2S in gases containing H2S and CO2 with an absorbent containing a diaminoether whose two secondary amine functions exhibit severe steric hindrance as defined above. The amine functions and the carbons of the main chain can be substituted by alkyl and hydroxyalkyl radicals.
Patent FR-2,934,172 describes the use of an absorbent solution based on a tertiary diamine in an acid compound removal method advantageously applied to the treatment of natural gas and combustion fumes, said amine being N,N,N′,N′-tetramethyl-1,6-hexanediamine.
Another limitation of the absorbent solutions commonly used in total deacidizing applications is too slow CO2 or COS capture kinetics. In cases where the desired CO2 or COS specifications level is very high, the fastest possible reaction kinetics is sought so as to reduce the height of the absorption column. Indeed, this equipment under pressure represents a significant part of the investment costs of the process.
Whether seeking maximum CO2 and COS capture kinetics in a total deacidizing application or minimum CO2 capture kinetics in a selective application, it is always desirable to use an absorbent solution having the highest cyclic capacity possible. This cyclic capacity, denoted by Δα, corresponds to the loading difference (a designates the number of moles of absorbed acid compounds nacid gas per kilogram of absorbent solution) between the absorbent solution discharged from the bottom of the absorption column and the absorbent solution fed to said column. Indeed, the higher the cyclic capacity of the absorbent solution, the lower the absorbent solution flow rate required for deacidizing the gas to be treated. In gas treatment methods, reduction of the absorbent solution flow rate also has a great impact on the reduction of investments, notably as regards absorption column sizing.
Another essential aspect of gas or industrial fumes treatment operations using a solvent remains the regeneration of the separation agent. Regeneration through expansion and/or distillation and/or entrainment by a vaporized gas referred to as “stripping gas” is generally considered depending on the absorption type (physical and/or chemical). The energy consumption required for solvent regeneration can be very high, which is in particular the case when the partial pressure of acid gases is low, and it can represent a considerable operating cost for the CO2 capture process.
It is well known to the person skilled in the art that the energy required for regeneration by distillation of an amine solution can be divided into three different items: the energy required for heating the absorbent solution between the top and the bottom of the regenerator, the energy required for lowering the acid gas partial pressure in the regenerator by vaporization of a stripping gas, and the enthalpy required for breaking the chemical bond between the amine and the CO2.
These first two items are proportional to the absorbent solution flows to be circulated in the plant in order to achieve a given specification. In order to decrease the energy consumption linked with the regeneration of the solvent, the cyclic capacity of the solvent is therefore once again preferably maximized. Indeed, the higher the cyclic capacity of the absorbent solution, the lower the absorbent solution flow rate required for deacidizing the gas to be treated.
There is therefore a need, in the field of gas deacidizing, for compounds that are good candidates for acid compounds removal from a gaseous effluent, notably, but not exclusively, selective removal of H2S over CO2, and that allow operation at lower operating costs (including the regeneration energy) and investment costs (including the cost of the absorption column).